Persist and prewarm agent tasks per thread (#17978)

## Summary
- persist registered agent tasks in the session state update stream so
the thread can reuse them
- prewarm task registration once identity registration succeeds, while
keeping startup failures best-effort
- isolate the session-side task lifecycle into a dedicated module so
AgentIdentityManager and RegisteredAgentTask do not leak across as many
core layers

## Testing
- cargo test -p codex-core startup_agent_task_prewarm
- cargo test -p codex-core
cached_agent_task_for_current_identity_clears_stale_task
- cargo test -p codex-core record_initial_history_

codex-rs/app-server-protocol/src/protocol/thread_history.rs

@@ -232,7 +232,9 @@ impl ThreadHistoryBuilder {
             RolloutItem::EventMsg(event) => self.handle_event(event),
             RolloutItem::Compacted(payload) => self.handle_compacted(payload),
             RolloutItem::ResponseItem(item) => self.handle_response_item(item),
-            RolloutItem::TurnContext(_) | RolloutItem::SessionMeta(_) => {}
+            RolloutItem::TurnContext(_)
+            | RolloutItem::SessionMeta(_)
+            | RolloutItem::SessionState(_) => {}
         }
     }